Image sensor having test pattern and offset correction method thereof

ABSTRACT

An image sensor includes a substrate including an active pixel and a test pattern, wherein the test pattern is located adjacent to the active pixel, wherein the active pixel comprises a first photodiode, a floating diffusion, a first channel provided between the first photodiode and the floating diffusion, and a first transfer gate electrode provided over the first channel, wherein the test pattern comprises a first test photodiode, a test floating diffusion, a second channel provided between the first test photodiode and the test floating diffusion, a first test transfer gate electrode provided over the second channel, and a first contact plug connected to the first test photodiode, and wherein the first test photodiode, the test floating diffusion, the second channel, and the first test transfer gate have substantially the same alignment errors as the first photodiode, the floating diffusion, the first channel, and the first transfer gate electrode, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Korean Patent Application No.10-2016-0097260, filed on Jul. 29, 2016, which is herein incorporated byreference in its entirety.

BACKGROUND 1. Field

Exemplary embodiments of the present invention relate to a semiconductordevice fabrication technology, and more particularly, to an image sensorhaving a test pattern for correcting an offset between pixels thereinand an offset correction method thereof.

2. Description of the Related Art

An image sensor is a device which converts an optical image into anelectrical signal. Recently, with the development of the computerindustry and the communication industry, the demand for an enhancedimage sensor has increased in various devices such as a digital camera,camcorder, Personal Communication System (PCS), game machine, securitycamera, medical micro-camera and a robot.

SUMMARY

Various embodiments are directed to an image sensor having a testpattern for correcting an offset between pixels therein and an offsetcorrection method thereof.

In an embodiment, an image sensor may include: a substrate Including anactive pixel and a test pattern, wherein the test pattern is locatedadjacent to the active pixel, wherein the active pixel comprises a firstphotodiode, a floating diffusion, a first channel provided between thefirst photodiode and the floating diffusion, and a first transfer gateelectrode provided over the first channel, wherein the test patterncomprises a first test photodiode, a test floating diffusion, a secondchannel provided between the first test photodiode and the test floatingdiffusion, a first test transfer gate electrode provided over the secondchannel, and a first contact plug connected to the first testphotodiode, and wherein the first test photodiode, the test floatingdiffusion, the second channel, and the first test transfer gate havesubstantially the same alignment errors as the first photodiode, thefloating diffusion, the first channel, and the first transfer gateelectrode, respectively.

The image sensor may further include: a logic circuit adjacent to theactive pixel; and a correction circuit formed in the logic circuit,wherein the correction circuit is connected to the first transfer gateelectrode. The test pattern may be formed between the logic circuit andthe active pixel. The test pattern may be connected to a side surface ofthe active pixel.

The first test photodiode, the test floating diffusion, the secondchannel, and the first test transfer gate electrode, in combination,form a test transfer transistor, wherein the correction circuit mayinclude a storage unit and a compensation unit, wherein the storage unitstores an electrical characteristic of the test transfer transistor, acorrection bias calculated based on the electrical characteristic, orboth; and wherein the compensation unit is connected to the storage unitand provides the correction bias to the first transfer gate electrode.The correction circuit may further include: a calculation unit connectedto the storage unit and processing the electrical characteristic of thetest transfer transistor. The correction circuit may further include: acomparison unit connected between the storage unit and the compensationunit and calculates the correction bias by comparing the electricalcharacteristic with a reference bias. The storage unit may include anonvolatile memory.

The active pixel may further include a second photodiode, a thirdchannel provided between the second photodiode and the floatingdiffusion, and a second transfer gate electrode provided over the thirdchannel, wherein the floating diffusion is arranged between the firstphotodiode and the second photodiode, wherein the test pattern regionfurther comprises a second test photodiode, a fourth channel providedbetween the second test photodiode and the test floating diffusion, asecond test transfer gate electrode provided over the fourth channel,and a second contact plug connected to the second test photodiode,wherein the test floating diffusion is arranged between the first testphotodiode and the second test photodiode, and wherein the second testphotodiode, the fourth channel, and the second test transfer gateelectrode have substantially the same alignment errors as the secondphotodiode, the third channel, and the second transfer gate electrode,respectively. The first channel has a first length, the second channelhas a second length, the third channel has a third length, and thefourth channel has a fourth length, wherein the second length of thesecond channel may be substantially equal to the first length of thefirst channel, and wherein the fourth length of the fourth channel maybe substantially equal to the third length of the third channel. Thethird length of the third channel may be smaller than the first lengthof the first channel.

The image sensor may further include: a third contact plug connected tothe test floating diffusion. The first test photodiode, the second testphotodiode, the test floating diffusion, the second channel, the fourthchannel, the first test transfer gate electrode, and the second testtransfer gate electrode comprise the same materials as the firstphotodiode, the second photodiode, the floating diffusion, the firstchannel, the third channel, the first transfer gate electrode, and thesecond transfer gate electrode, respectively.

In an embodiment, there is provided an offset correction method of animage sensor having an active pixel region and a test pattern region,each of which includes a plurality of unit pixel groups arranged in atwo-dimensional manner, the unit pixel groups each including a pluralityof pixels sharing one floating diffusion. The offset correction methodmay include: measuring electrical characteristics of test transfertransistors in the test pattern region; processing electricalcharacteristics of test transfer transistors formed at first positionsin the unit pixel groups of the test pattern region; calculatingcorrection values by comparing the processed electrical characteristicsto reference characteristics of transfer transistors formed at secondpositions in the unit pixel groups of the active pixel region; andcorrecting offsets for the transfer transistors formed at the secondpositions in the unit pixel groups of the active pixel region based onthe calculated correction value.

The first position and the second position in each of the unit pixelgroups may be the same positions. The active pixel region may include afirst channel between a first photodiode and a floating diffusion, and afirst transfer gate electrode over the first channel, the test patternregion may include a second channel between a first test photodiode anda test floating diffusion, a first test transfer gate electrode over thesecond channel, and a first contact plug connected to the first testphotodiode, and the first test photodiode, the test floating diffusion,the second channel and the first test transfer gate electrode may havesubstantially the same alignment errors as the first photodiode, thefloating diffusion, the first channel and the first transfer gateelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 are layout diagrams for describing an image sensor inaccordance with an embodiment.

FIGS. 4 and 5 are cross-sectional views for describing main componentsof the image sensor in accordance with the embodiment.

FIGS. 6 and 7 are cross-sectional views for describing the maincomponents of the image sensor in accordance with the embodiment.

FIG. 8 is a potential distribution diagram of a transfer transistor inaccordance with the present embodiment.

FIG. 9 is a block diagram illustrating a correction circuit inaccordance with an embodiment.

FIG. 10 is a flowchart for describing an offset correction method of animage sensor in accordance with an embodiment.

DETAILED DESCRIPTION

Various embodiments will be described below in more detail withreference to the accompanying drawings. The present invention may,however, be embodied in different forms and should not be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete, andwill fully convey the scope of the present invention to those skilled inthe art. Throughout the disclosure, like reference numerals refer tolike parts throughout the various figures and embodiments of the presentinvention.

The terms used in this specification are used only for describingexemplary embodiments while not limiting the present invention. Theterms of a singular form may include plural forms unless referred to thecontrary. The meaning of ‘comprise’ or ‘comprising’ used in thespecification specifies a component, a step, an operation, and/or anelement, but does not exclude other components, steps, operations,and/or elements.

When one element is referred to as being “connected to” or “coupled to”another element, it may indicate that the former element is directlyconnected or coupled to the latter element or another element isinterposed therebetween. Moreover, when one element is referred to asbeing “directly connected to” or “directly coupled to” another element,it may indicate that no element is interposed therebetween. Furthermore,“and/or” includes each of the described items and one or morecombinations of the items.

The terms such as ‘below’, ‘beneath’, ‘lower’, ‘above’ and ‘upper’,which are spatially relative terms, may be used to describe thecorrelation between one element or components and another element orother components as illustrated in the drawings. A spatially relativeterm should be understood as a term including different directions of anelement when the element is used or operated, in addition to thedirection illustrated in the drawings. For example, when an elementillustrated in a drawing is turned over, the element which is referredto as being ‘below’ or ‘beneath’ another element may be placed aboveanother element.

Moreover, embodiments included in this specification will be describedwith reference to cross-sectional views and/or plane views as idealexemplary views of the present invention. In the drawings, thedimensions of layers and regions may be exaggerated for clarity ofillustration. Accordingly, the shapes of the exemplary views may bemodified according to fabrication techniques and/or allowable errors.Therefore, the present embodiments are not limited to the specificshapes illustrated in the exemplary views, but may include other shapesthat may be created according to fabrication processes. For example, aregion illustrated as angular may have a rounded shape or a certainradius of curvature. Regions exemplified in the drawings have generalproperties, and the shapes of the regions are used to illustratespecific shapes of elements. Thus, this should not be construed aslimited to the scope of the present invention.

Throughout the specification, like reference numerals refer to the sameelements. Therefore, although the same or similar reference numerals arenot mentioned or described in the corresponding drawing, the referencenumerals may be described with reference to other drawings. Furthermore,although elements are not represented by reference numerals, theelements may be described with reference to other drawings.

In the present specification, source electrodes and drain electrodes oftransistors may be compatible with each other. Since transistorsdescribed in this specification are Field Effect Transistors (FETs) suchMOS transistors, the source electrodes and the drain electrodes may besymmetric and compatible with each other. Therefore, in a variety ofembodiments, a source electrode may be analyzed as a drain electrode,and a drain electrode may be analyzed as a source electrode.

The following embodiments provide a test pattern for correcting anoffset between pixels in an image sensor and an offset correction methodusing the same. The offset between pixels may indicate that transfertransistors included in a plurality of pixels, respectively, are notuniformly formed due to an error of the fabrication process, orvariations of the transfer transistors. Hereafter, embodiments will bedescribed in detail with reference to the accompanying drawings.

FIGS. 1 to 3 are layout diagrams for describing an image sensor inaccordance with an embodiment. Referring to FIG. 1, a plurality of chips13 and scribe lanes SC may be formed on a substrate 11. The substrate 11may include a semiconductor substrate such as a silicon wafer. Forexample, the substrate 11 may include single crystal silicon containingP-type impurities. The plurality of chips 13 may be two-dimensionallyarranged in or over the substrate 11. The respective chips 13 may beseparated from each other by the scribe lanes SC.

Referring to FIG. 2, the chip 13 may include an active pixel region 15,a test pattern region 17, a logic circuit region 18 and a correctioncircuit 19. FIG. 2 illustrates a part of FIG. 1 in detail.

The logic circuit region 18 may be formed to surround the active pixelregion 15. The test pattern region 17 may be formed adjacent to a sideof the active pixel region 15. The test pattern region 17 may be formedbetween the active pixel region 15 and the logic circuit region 18. Thecorrection circuit 19 may be formed in the logic circuit region 18. Theactive pixel region 15 and the test pattern region 17 may be formed onthe same wafer. The logic circuit region 18 and the correction circuit19 may be formed on a wafer different from the wafer where the activepixel region 15 and the test pattern region 17 are formed. Furthermore,the respective elements may be vertically stacked.

In an embodiment, the test pattern region 17 may be connected to a sidesurface of the active pixel region 15. The test pattern region 17 may beconnected to an edge of the active pixel region 15. The test patternregion 17 may be directly contacted with the side surface of the activepixel region 15.

In accordance with the present embodiment, the test pattern region 17may be formed at a position adjacent to the active pixel region 15. Thetest pattern region 17 may include test transfer transistors forexample, Tx1 t and Tx2 t of FIG. 6, which exhibit substantially the samestructure and electrical characteristic as transfer transistors of theactive pixel region 15 for example, Tx1 and Tx2 of FIG. 6. Due to thesame structure, the test pattern region 17 may have substantially thesame alignment error as the active pixel region 15. The accuracy ofcorrection biases for example, Vp1 and Vp2 of FIG. 6, which arecalculated based on the electrical characteristics measured through thetest transfer transistors for example, Tx1 t and Tx2 t of FIG. 6, may bemore excellent than in the related art. Thus, the electricalcharacteristics of the transfer transistors for example, Tx1 and Tx2 ofFIG. 6 may be optimized, which makes it possible to implement an imagesensor having satisfactory electrical characteristics.

In an embodiment, the test pattern region 17 may be formed at variouspositions near the active pixel region 15 in the chip 13. For example,the test pattern region 17 may be formed adjacent to two side surfacesof the active pixel region 15 or formed to surround four surfaces of theactive pixel region 15.

FIG. 3 illustrates an image sensor in accordance with the presentembodiment which has a 4-shared pixel structure. Referring to FIG. 3,the active pixel region 15 may include a first photodiode 31, a secondphotodiode 32, a third photodiode 33, a fourth photodiode 34, a firsttransfer gate electrode 41, a second transfer gate electrode 42, a thirdtransfer gate electrode 43, a fourth transfer gate electrode 44 and afloating diffusion 47. The test pattern region 17 may include a firsttest photodiode 131, a second test photodiode 132, a third testphotodiode 133, a fourth test photodiode 134, a first test transfer gateelectrode 141, a second test transfer gate electrode 142, a third testtransfer gate electrode 143, a fourth test transfer gate electrode 144,a test floating diffusion 147, a first contact plug 151, a secondcontact plug 152, a third contact plug 153, a fourth contact plug 154and a fifth contact plug 157.

The first photodiode 31, the second photodiode 32, the third photodiode33 and the fourth photodiode 34 may be separated from one another. Thefloating diffusion 47 may be surrounded by the first photodiode 31, thesecond photodiode 32, the third photodiode 33 and the fourth photodiode34. The first photodiode 31 may be formed adjacent to the third andfourth photodiodes 33 and 34, while facing the second photodiode 32. Thethird photodiode 33 may be formed adjacent to the first and secondphotodiodes 31 and 32, while facing the fourth photodiode 34.

The first transfer gate electrode 41 may be formed between the floatingdiffusion 47 and the first photodiode 31. The second transfer gateelectrode 42 may be formed between the floating diffusion 47 and thesecond photodiode 32. The third transfer gate electrode 43 may be formedbetween the floating diffusion 47 and the third photodiode 33. Thefourth transfer gate electrode 44 may be formed between the floatingdiffusion 47 and the fourth photodiode 34.

The first photodiode 31, the second photodiode 32, the third photodiode33 and the fourth photodiode 34 may share the floating diffusion 47. Forexample, the first photodiode 31, the second photodiode 32, the thirdphotodiode 33 and the fourth photodiode 34 may correspond to colors, Gr,Gb, B and R, respectively.

The plurality of 4-shared pixel structures may be repetitively arrangedin the row and column directions within the active pixel region 15. Theplurality of photodiodes 31 to 34, the plurality of transfer gateelectrodes 41 to 44 and the plurality of floating diffusions 47 may berepetitively arranged in the row and column directions within the activepixel region 15.

The first to fourth test photodiodes 131 to 134, the first to fourthtest transfer gate electrodes 141 to 144, and the test floatingdiffusion 147 may correspond to the first to fourth photodiodes 31 to34, the first to fourth transfer gate electrodes 41 to 44, and thefloating diffusion 47, respectively. The first to fourth testphotodiodes 131 to 134, the first to fourth test transfer gateelectrodes 141 to 144, and the test floating diffusion 147 may be formedsubstantially at the same time as the first to fourth photodiodes 31 to34, the first to fourth transfer gate electrodes 41 to 44, and thefloating diffusion 47, and include substantially the same materials andstructures as the first to fourth photodiodes 31 to 34, the first tofourth transfer gate electrodes 41 to 44, and the floating diffusion 47,respectively.

The first contact plug 151 may be connected to the first test photodiode131. The second contact plug 152 may be connected to the second testphotodiode 132. The third contact plug 153 may be connected to the thirdtest photodiode 133. The fourth contact plug 154 may be connected to thefourth test photodiode 134. The fifth contact plug 157 may be connectedto the test floating diffusion 147.

The first test photodiode 131, the second test photodiode 132, the thirdtest photodiode 133 and the fourth test photodiode 134 may be separatedfrom one another. The test floating diffusion 147 may be surrounded bythe first test photodiode 131, the second test photodiode 132, the thirdtest photodiode 133 and the fourth test photodiode 134. The first testphotodiode 131 may be formed adjacent to the third and fourth testphotodiodes 133 and 134, while facing the second test photodiode 132.The third test photodiode 133 may be formed adjacent to the first andsecond test photodiodes 131 and 132, while facing the fourth testphotodiode 134.

The first test transfer gate electrode 141 may be formed between thetest floating diffusion 147 and the first test photodiode 131. Thesecond test transfer gate electrode 142 may be formed between the testfloating diffusion 147 and the second test photodiode 132. The thirdtest transfer gate electrode 143 may be formed between the test floatingdiffusion 147 and the third test photodiode 133. The fourth testtransfer gate electrode 144 may be formed between the test floatingdiffusion 147 and the fourth test photodiode 134. The first testphotodiode 131, the second test photodiode 132, the third testphotodiode 133 and the fourth test photodiode 134 may share the testfloating diffusion 147.

The plurality of test photodiodes 131 to 134, the plurality of testtransfer gate electrodes 141 to 144, the plurality of test floatingdiffusion s147 and the plurality of contact plugs 151 to 154 and 157 maybe repetitively arranged in the test pattern region 17. For example, 20or more test photodiodes 131 to 134, 20 or more test transfer gateelectrodes 141 to 144, 20 or more test floating diffusions 147 and 20 ormore contact plugs 151 to 154 and 157 may be repetitively formed in thetest pattern region 17. In an embodiment, the first to fourth transfergate electrodes 41 to 44 and the first to fourth test transfer gateelectrodes 141 to 144 may have various sizes and shapes.

Although the image sensor in accordance with the present embodiment hasthe 4-shared pixel structure, an image sensor in accordance with anotherembodiment may include a 2-shared pixel structure, a 3-shared pixelstructure, a 4-shared pixel structure, an 8-shared pixel structure, anN-shared pixel structure, or a combination thereof.

FIGS. 4 and 5 are cross-sectional views taken along the lines I-I′ andII-II′ of FIG. 3, respectively. Referring to FIG. 4, the active pixelregion 15 may include a substrate 11, an isolation layer 21, a gatedielectric layer 23, the first photodiode 31, the second photodiode 32,the first transfer gate electrode 41, the second transfer gate electrode42, the floating diffusion 47, a first Insulating layer 63, a colorfilter layer 72 and a microlens layer 75.

The substrate 11 may have first and second channels CH1 and CH2 definedtherein. The first channel CH1 may have a first length L1, and thesecond channel CH2 may have a second length L2. The first photodiode 31,the second photodiode 32, the floating diffusion 47, the first channelCH1, and the second channel CH2 may be defined in the substrate 11 bythe isolation layer 21.

The floating diffusion 47 may be formed between the first and secondphotodiodes 31 and 32. The first channel CH1 may be formed between thefloating diffusion 47 and the first photodiode 31. The second channelCH2 may be formed between the floating diffusion 47 and the secondphotodiode 32. The first transfer gate electrode 41 may be formed overthe first channel CH1. The second transfer gate electrode 42 may beformed over the second channel CH2. The gate dielectric layer 23 may beinterposed between the first transfer gate electrode 41 and the firstchannel CH1 and between the second transfer gate electrode 42 and thesecond channel CH2.

An alignment error may occur among the first photodiode 31, the secondphotodiode 32, the first transfer gate electrode 41, the second transfergate electrode 42, the floating diffusion 47, the first channel CH1 andthe second channel CH2. The first photodiode 31, the second photodiode32, the first transfer gate electrode 41, the second transfer gateelectrode 42, the floating diffusion 47, the first channel CH1 and thesecond channel CH2 may be misaligned by an alignment error occurring ina lithography process. For example, the first length L1 of the firstchannel CH1 may be different from the second length L2 of the secondchannel CH2. For example, the second length L2 may be smaller than thefirst length L1.

The first insulating layer 63 may cover an upper surface of thesubstrate 11. The first insulating layer 63 may cover the isolationlayer 21, the first photodiode 31, the second photodiode 32, the firsttransfer gate electrode 41, the second transfer gate electrode 42, andthe floating diffusion 47.

The color filter layer 72 may be formed on a rear surface of thesubstrate 11. The microlens layer 75 may be formed on the color filterlayer 72. The color filter layer 72 may be arranged between themicrolens layer 75 and the first photodiode 31 and between the microlenslayer 75 and the second photodiode 32. The color filter layer 72 mayinclude colors, Gr, Gb, B and R.

The image sensor in accordance with the present embodiment may becharacterized as a Back Side Illumination (BSI) image sensor. Thesubstrate 11 containing P-type impurities may be interposed between themicrolens layer 75 and the first photodiode 31 and between the microlenslayer 75 and the second photodiode 32. The first and second photodiodes31 and 32 may directly contact the substrate 11 containing P-typeimpurities.

The test pattern region 17 may include the substrate 11, the isolationlayer 21, the gate dielectric layer 23, the first test photodiode 131,the second test photodiode 132, the first test transfer gate electrode141, the second test transfer gate electrode 142, the test floatingdiffusion 147, the first contact plug 151, the second contact plug 152,the fifth contact plug 157 and the first insulating layer 63. Thesubstrate 11 may have first and second test channels CH1 t and CH2 tdefined therein. The first test channel CH1 t may have a third lengthL3, and the second test channel CH2 t may have a fourth length L4.

The first test photodiode 131, the second test photodiode 132, the firsttest transfer gate electrode 141, the second test transfer gateelectrode 142, the test floating diffusion 147, the first test channelCH1 t and the second test channel CH2 t may correspond to the firstphotodiode 31, the second photodiode 32, the first transfer gateelectrode 41, the second transfer gate electrode 42, the floatingdiffusion 47, the first channel CH1 and the second channel CH2,respectively. The first test photodiode 131, the second test photodiode132, the first test transfer gate electrode 141, the second testtransfer gate electrode 142, the test floating diffusion 147, the firsttest channel CH1 t and the second test channel CH2 t may be formedsubstantially at the same time as the first photodiode 31, the secondphotodiode 32, the first transfer gate electrode 41, the second transfergate electrode 42, the floating diffusion 47, the first channel CH1 andthe second channel CH2, and include substantially the same materials andstructures as the first photodiode 31, the second photodiode 32, thefirst transfer gate electrode 41, the second transfer gate electrode 42,the floating diffusion 47, the first channel CH1 and the second channelCH2, respectively.

The first test photodiode 131, the second test photodiode 132, the firsttest transfer gate electrode 141, the second test transfer gateelectrode 142, the test floating diffusion 147, the first test channelCH1 t and the second test channel CH2 t may include substantially thesame alignment errors as the first photodiode 31, the second photodiode32, the first transfer gate electrode 41, the second transfer gateelectrode 42, the floating diffusion 47, the first channel CH1 and thesecond channel CH2, respectively. For example, the first test photodiode131, the second test photodiode 132, the first test transfer gateelectrode 141, the second test transfer gate electrode 142, the testfloating diffusion 147, the first test channel CH1 t and the second testchannel CH2 t may be misaligned during a lithography process by as muchas the first photodiode 31, the second photodiode 32, the first transfergate electrode 41, the second transfer gate electrode 42, the floatingdiffusion 47, the first channel CH1 and the second channel CH2,respectively.

The third length L3 of the first test channel CH1 t may be differentfrom the fourth length L4 of the second test channel CH2 t. For example,the fourth length L4 may be smaller than the third length L3. The thirdlength L3 may be substantially equal to the first length L1, and thefourth length L4 may be substantially equal to the second length L2. Thefirst test photodiode 131, the second test photodiode 132, the testfloating diffusion 147, the first test channel CH1 t and the second testchannel CH2 t may be defined in the substrate 11 by the isolation layer21.

The test floating diffusion 147 may be formed between the first andsecond test photodiodes 131 and 132. The first test channel CH1 t may beformed between the test floating diffusion 147 and the first testphotodiode 131. The second test channel CH2 t may be formed between thetest floating diffusion 147 and the second test photodiode 132. Thefirst test transfer gate electrode 141 may be formed over the first testchannel CH1 t. The second test transfer gate electrode 142 may be formedover the second test channel CH2 t. The gate dielectric layer 23 may beinterposed between the first test transfer gate electrode 141 and thefirst test channel CH1 t and between the second test transfer gateelectrode 142 and the second test channel CH2 t.

The first insulating layer 63 may cover the upper surface of thesubstrate 11. The first insulating layer 63 may cover the isolationlayer 21, the first test photodiode 131, the second test photodiode 132,the first test transfer gate electrode 141, the second test transfergate electrode 142 and the test floating diffusion 147.

The first contact plug 151 may be formed on the first test photodiode131, the second contact plug 152 may be formed on the second testphotodiode 132, and the fifth contact plug 157 may be formed on the testfloating diffusion 147. The first contact plug 151 may be connected tothe first test photodiode 131 through the first insulating layer 63. Thesecond contact plug 152 may be connected to the second test photodiode132 through the first insulating layer 63. The fifth contact plug 157may be connected to the test floating diffusion 147 through the firstinsulating layer 63.

In an embodiment, the color filter layer 72 and the microlens layer 75may be formed on the rear surface of the substrate 11 in the testpattern region 17.

Referring to FIG. 5, each of the first photodiode 31, the secondphotodiode 32, the first test photodiode 131, and the second testphotodiode 132 may include first and second Impurity regions 25 and 27.The first impurity region 25 may include N-type impurities. The secondimpurity region 27 may include P-type impurities. The second impurityregion 27 may be formed between the first impurity region 25 and thefirst insulating layer 63. The second impurity region 27 may be directlycontacted with the first impurity region 25. Each of the first andsecond contact plugs 151 and 152 may directly contact the first impurityregion 25 through the first insulating layer 63 and the second impurityregion 27. The fifth contact plug 157 may directly contact the testfloating diffusion 147 through the first insulating layer 63.

In the active pixel region 15, the color filter layer 72 may be formedon the first insulating layer 63. The microlens layer 75 may be formedon the color filter layer 72. In the test pattern region 17, the colorfilter layer 72, the microlens layer 75 and the third insulating layer79 may be omitted. The first contact plug 151, the second contact plug152, and the fifth contact plug 157 may be exposed on the firstinsulating layer 63.

Hereafter, a method for correcting an offset between pixels in an imagesensor having the above-described structure using the test patternregion will be described in detail.

FIGS. 6 and 7 are cross-sectional views illustrating main components fordescribing the method for correcting an offset between pixels in theimage sensor in accordance with the embodiment. FIGS. 6 and 7 arecross-sectional views taken along the lines I-I′ and II-II′ of FIG. 3.Referring to FIG. 6, the image sensor in accordance with the presentembodiment may include the active pixel region 15, the test patternregion 17, and the correction circuit 19. The correction circuit 19 maybe referred to as a transfer transistor correction circuit.

The active pixel region 15 may include the substrate 11, the firstphotodiode 31, the second photodiode 32, the first transfer gateelectrode 41, the second transfer gate electrode 42, the floatingdiffusion 47, a reset transistor Rx, a drive transistor Dx, and a selecttransistor Sx. In the substrate 11, the first channel CH1 and the secondchannel CH2 may be defined by the first transfer gate electrode 41 andthe second transfer gate electrode 42, respectively. The first channelCH1 may be formed between the first photodiode 31 and the floatingdiffusion 47 and under the first transfer gate electrode 41, and havethe first length L1. The second channel CH2 may be formed between thesecond photodiode 32 and the floating diffusion 47 and under the secondtransfer gate electrode 42, and have the second length L2. The firsttransfer gate electrode 41 and the first channel CH1 may constitute thefirst transfer transistor Tx1. The second transfer gate electrode 42 andthe second channel CH2 may constitute a second transfer transistor Tx2.

The test pattern region 17 may include the substrate 11, the first testphotodiode 131, the second test photodiode 132, the first test transfergate electrode 141, the second test transfer gate electrode 142, thetest floating diffusion 147, the first contact plug 151, the secondcontact plug 152 and the fifth contact plug 157. In the substrate 11,the first test channel CH1 t and the second test channel CH2 t may bedefined by the first test transfer gate electrode 141 and the secondtest transfer gate electrode 142, respectively.

The first test channel CH1 t may be formed between the first testphotodiode 131 and the test floating diffusion 147 and under the firsttest transfer gate electrode 141, and have the third length L3. Thesecond test channel CH2 t may be formed between the second testphotodiode 132 and the test floating diffusion 147 and under the secondtest transfer gate electrode 142, and have the fourth length L4. Thefirst test transfer gate electrode 141 and the first test channel CH1 tmay constitute a first test transfer transistor Tx1 t. The second testtransfer gate electrode 142 and the second test channel CH2 t mayconstitute a second test transfer transistor Tx2 t.

The floating diffusion 47 may be formed between the first and secondphotodiodes 31 and 32. That is, the first and second photodiodes 31 and32 may share the floating diffusion 47. The first channel CH1 may bedefined between the floating diffusion 47 and the first photodiode 31.The second channel CH2 may be defined between the floating diffusion 47and the second photodiode 32. The first transfer gate electrode 41 maybe formed over the first channel CH1. The second transfer gate electrode42 may be formed over the second channel CH2. Each of the first andsecond transfer gate electrodes 41 and 42 may be connected to thecorrection circuit 19.

The first test photodiode 131, the second test photodiode 132, the firsttest transfer gate electrode 141, the second test transfer gateelectrode 142, the test floating diffusion 147, the first test channelCH1 t and the second test channel CH2 t may be formed substantially atthe same time as the first photodiode 31, the second photodiode 32, thefirst transfer gate electrode 41, the second transfer gate electrode 42,the floating diffusion 47, the first channel CH1 and the second channelCH2, and include substantially the same materials and structures as thefirst photodiode 31, the second photodiode 32, the first transfer gateelectrode 41, the second transfer gate electrode 42, the floatingdiffusion 47, the first channel CH1 and the second channel CH2,respectively. That is, the pixels of the test pattern region 17 may havethe same offsets as the transfer transistors included in the respectivepixels of the active pixel region 15.

The first test photodiode 131, the second test photodiode 132, the firsttest transfer gate electrode 141, the second test transfer gateelectrode 142, the test floating diffusion 147, the first test channelCH1 t and the second test channel CH2 t may be misaligned tosubstantially the same degree as the first photodiode 31, the secondphotodiode 32, the first transfer gate electrode 41, the second transfergate electrode 42, the floating diffusion 47, the first channel CH1 andthe second channel CH2, respectively. The second length L2 of the secondchannel CH2 may be smaller than the first length L1 of the first channelCH1. The third length L3 of the first test channel CH1 t may besubstantially equal to the first length L1, and the fourth length L4 ofthe second test channel CH2 t may be substantially equal to the secondlength L2. That is, the first test transfer transistor Tx1 t may havesubstantially the same offset as the first transfer transistor Tx1, andthe second test transfer transistor Tx2 may have substantially the sameoffset as the second transfer transistor Tx2.

The electrical characteristic of the first test transfer transistor Tx1t may be measured by applying a first source bias Vs1 to the firstcontact plug 151, applying a drain bias Vd to the fifth contact plug157, and applying a first gate bias Vg1 to the first test transfer gateelectrode 141. The electrical characteristic of the second test transfertransistor Tx2 t may be measured by applying a second source bias Vs2 tothe second contact plug 152, applying the drain bias Vd to the fifthcontact plug 157, and applying a second gate bias Vg1 to the second testtransfer gate electrode 142.

The first test transfer transistor Tx1 t including the first testtransfer gate electrode 141 and the first test channel CH1 t may havesubstantially the same electrical characteristics as the first transfertransistor Tx1 including the first transfer gate electrode 41 and thefirst channel CH1. The second test transfer transistor Tx2 t includingthe second test transfer gate electrode 142 and the second test channelCH2 t may have substantially the same electrical characteristic as thesecond transfer transistor Tx2 including the second transfer gateelectrode 42 and the second channel CH2.

Based on the electrical characteristics measured in the first and secondtest transfer transistors Tx1 t and Tx2 t, a first correction bias Vp1for the first transfer gate electrode 41 and a second correction biasVp2 for the second transfer gate electrode 42 may be calculated. Theelectrical characteristics of the first test transfer transistor Tx1 t,the electrical characteristics of the second test transfer transistorTx2 t, the first correction bias Vp1, the second correction bias Vp2, ora combination thereof may be written to the correction circuit 19. Thecorrection circuit 19 may include a nonvolatile memory such as One TimeProgrammable (OTP) memory. The correction circuit 19 may be connected tothe first and second transfer gate electrodes 41 and 42. The correctioncircuit 19 may serve to apply the first correction bias Vp1 to the firsttransfer gate electrode 41, and apply the second correction bias Vp2 tothe second transfer gate electrode 42.

In an embodiment, the first contact plug 151, the second contact plug152 and the fifth contact plug 157 may correspond to conceptual elementsfor describing lines capable of applying power for checking theelectrical characteristics of the test transfer transistors Tx1 t andTx2 t.

In accordance with the present embodiment, the electricalcharacteristics of the first and second transfer transistors Tx1 and Tx2can be optimized and make it possible to implement an image sensorhaving excellent electrical characteristics. Specifically, variations ofthe transfer transistors Tx1 and Tx2 formed in the active pixel region15 can be corrected through the test transfer transistors which areformed in the test pattern region 17 to correspond to the transfertransistors. Thus, it is possible to prevent characteristic degradationwhich may occur due to the variations of the transfer transistors, forexample, an image lag.

Referring to FIG. 7, the second length L2 of the second channel CH2 maybe substantially equal to the first length L1 of the first channel CH1.The third length L3 of the first test channel CH1 t may be substantiallyequal to the first length L1, and the fourth length L4 of the secondtest channel CH2 t may be substantially equal to the second length L2.

FIG. 8 is a potential distribution diagram for describing the operationof the transfer transistor in accordance with the present embodiment. InFIG. 8, ‘Φ’ represents a potential barrier. Referring to FIG. 8,photocharges generated in response to incident light, that is, electronsmay be accumulated in the photodiodes 31 and 32.

While the first transfer transistor Tx1 has a low surface potential, thefirst transfer transistor Tx1 may have a high potential barrier Φ_(Tx1).Thus, it is possible to prevent the electrons stored in the firstphotodiode 31 from migrating to adjacent pixels during an integrationtime in which the first photodiode 31 generates photocharges in responseto incident light, thereby securing sufficient capacitance. That is, alarge number of electrons may be stored in the first photodiode 31.

However, when the photocharges generated by the first photodiode 31 aretransferred to the floating diffusion 47 in response to a signal appliedto the first transfer transistor Tx1, all of the electrons accumulatedin the first photodiode 31 may not be transferred to the floatingdiffusion 47 due to the high potential barrier Φ_(Tx1) of the firsttransfer transistor Tx1. In this case, image lag may occur. When thethreshold voltage of the first transfer transistor Tx1 is lowered, thepotential barrier Φ_(Tx1) may be lowered. Then, all of the electronsaccumulated in the first photodiode 31 may migrate to the floatingdiffusion 47, which makes it possible to prevent the occurrence of imagelag.

While the second transfer transistor Tx2 has a high surface potential,the second transfer transistor Tx2 may have a low potential barrierΦ_(Tx2). When the second transfer transistor Tx2 has a low potentialbarrier Φ_(Tx2), the electrons accumulated in the second photodiode 32may easily migrate to the floating diffusion 47, but the capacitance ofthe second photodiode 32 may be reduced. Then, a sufficient number ofelectrons cannot be accumulated in the second photodiode 32 during theintegration time. When the threshold voltage of the second transfertransistor Tx2 is raised, the potential barrier Φ_(Tx2) may be raised toaccumulate a larger number of electrons in the second photodiode 32during the integration time.

Each of the transfer transistors Tx1 and Tx2 may include a variationcaused by an error of the fabrication process. Thus, the transfertransistors Tx1 and Tx2 may have different surface potentials.Therefore, only globally controlling the biases applied to the transfertransistors Tx1 and Tx2 has a limitation in optimizing the performanceof the transfer transistors Tx1 and Tx2.

In accordance with the present embodiment, however, the first correctionbias Vp1 for the first transfer gate electrode 41 and the secondcorrection bias Vp2 for the second transfer gate electrode 42 may beapplied to optimize the performances of the transfer transistors Tx1 andTx2. The correction biases Vp1 and Vp2 may be calculated based on theelectrical characteristics of the test transfer transistors Tx1 t andTx2 t. Through the correction biases Vp1 and Vp2, the performance of thetransfer transistors Tx1 and Tx2 having different potentials can beoptimized.

FIG. 9 is a block diagram illustrating a correction circuit inaccordance with the present embodiment. FIG. 10 is a flowchart fordescribing an offset correction method of an image sensor in accordancewith an embodiment. Referring to FIG. 9, the correction circuit 19 inaccordance with the present embodiment may include a calculation unit191, a storage unit 193, a comparison unit 195 and a compensation unit197.

The calculation unit 191 may statistically process the electricalcharacteristics of the test transfer transistors Tx1 t and Tx2 t in thetest pattern region 17. For example, unit pixel groups, in each of whichfour pixels or photodiodes share one floating diffusion FD, aretwo-dimensionally arranged in the test pattern region 17. Thecalculation unit 191 may measure the electrical characteristics of thefirst test transfer transistors in the respective unit pixel groups, forexample, the threshold voltages, and calculate an average thresholdvoltage of the first test transfer transistors.

Referring to FIG. 3, the first test transfer transistor may correspondto the test transfer transistor which is formed at the left top in eachof the unit pixel groups. That is, the first test transfer transistormay correspond to the first test transfer transistor Tx1 t. Theelectrical characteristics of the first test transfer transistors, whichare measured in the unit pixel groups of the test pattern region 17, maybe used as data to compensate for offsets of the first transfertransistors of the unit pixel groups in the active pixel region 15,which have substantially the same characteristics. Referring to FIG. 3,the first transfer transistor may correspond to a transfer transistorwhich is formed at the left top in each of the unit pixel groups. Thatis, the first transfer transistor may correspond to the first transfertransistor Tx1. The calculation unit 191 may be connected to the storageunit 193.

The storage unit 193 may serve to store the electrical characteristicsof the test transfer transistors Tx1 t and Tx2 t, the correction biasesVp1 and Vp2 calculated by the comparison unit 195 based on theelectrical characteristics, or both. The storage unit 193 may include anonvolatile memory such as OTP memory.

The comparison unit 195 may be connected between the storage unit 193and the compensation unit 197. The comparison unit 195 may calculate thecorrection biases Vp1 and Vp2 by comparing the electrical characteristicdata measured and processed by the calculation unit 191 with a presetreference bias. For example, the correction voltages Vp1 and Vp2 may beobtained by comparing the average threshold voltages of the testtransfer transistors Tx1 t and Tx2 t of the test pattern region 17 witha reference voltage preset to drive the transfer transistors Tx1 and Tx2of the active pixel region 15. The average threshold voltages of thetest transfer transistors Tx1 t and Tx2 t of the test pattern region 17may be calculated through the calculation unit 191.

The compensation unit 197 may be connected to the storage unit 193 andprovides the correction biases Vp1 and Vp2 to the transfer gateelectrodes 41 and 42 in the active pixel region 15.

In an embodiment, the correction circuit 19 may include the storage unit193, the comparison unit 195 and the compensation unit 197 and may notinclude the calculation unit 191. In another embodiment, the correctioncircuit 19 may include the storage unit 193 and the compensation unit197 and may not include the comparison unit 195.

Referring to FIG. 10, the offset correction method of the image sensormay include measuring electrical characteristics of test transfertransistors at step B01, selectively processing the measured electricalcharacteristics at step B02, calculating a correction value by comparingthe processed electrical characteristics with a reference characteristicat step B04, and correcting an offset between pixels in the active pixelregion based on the calculated correction value at step B06. In anembodiment, each of the active pixel region 15 and the test patternregion 17 includes a plurality of unit pixel groups arranged in atwo-dimensional manner, and the plurality of unit pixel groups eachincluding four pixels sharing one floating diffusion FD.

Electrical characteristics of the test transfer transistors Tx1 t andTx2 t may be measured in each of the unit pixel groups of the testpattern region 17 at step B01. For example, the threshold voltages ofthe test transfer transistors Tx1 t and Tx2 t may be measured in each ofthe unit pixel groups. Specifically, referring to FIGS. 6 and 7, theelectrical characteristics of the test transfer transistors Tx1 t andTx2 t may be measured by applying a first source voltage Vs1 to thefirst contact plug 151, applying a second source voltage Vs2 to thesecond contact plug 152, applying a drain voltage Vd to the fifthcontact plug 157, applying a first gate voltage Vg1 to the first testtransfer gate electrode 141, and applying a second gate voltage Vg2 tothe second test transfer gate electrode 142 at a fab-out step or waferlevel probe test step of the substrate 11.

Then, the electrical characteristics measured by the calculation unit191 may be selectively processed at step B02. Specifically, the measureddata of the test transfer transistors formed at the same positions inthe respective unit pixel groups of the test pattern region 17 may beextracted and processed. For example, all of the threshold voltages ofthe test transfer transistors Tx1 t positioned at the left tops of therespective unit pixel groups may be added up to calculate an averagethereof.

The above-described process may be repeated to calculate the averagethreshold voltages of the test transfer transistors positioned at theleft bottoms, the right tops, and the right bottoms of the respectiveunit pixel groups. The processed electrical characteristics may be usedas data to compensate for the offsets of the transfer transistors whichare positioned at the same positions in the respective unit pixel groupsof the active pixel region 15 and have substantially the samecharacteristics. The electrical characteristics processed by thecalculation unit 191 may be stored in the storage unit 193 at step B03.

Then, the comparison unit 195 may calculate a correction value bycomparing the processed electrical characteristic with a referencecharacteristic at step B04, and the compensation unit 197 may compensatefor an offset between pixels of the active pixel region 15 based on thecorrection value calculated by the comparison unit at step B06. Thecorrection value calculated by the comparison unit 195 may be stored inthe storage unit 193 at step B05.

For example, the processed electrical characteristic may correspond toan average threshold voltage value, and the reference characteristic maycorrespond to threshold voltages of the transfer transistors Tx1 and Tx2of the active pixel region 15. The correction value may include thecorrection biases Vp1 and Vp2 obtained by reflecting the averagethreshold voltage into the reference voltages which are preset to drivethe transfer transistors Tx1 and Tx2 of the active pixel region 15.Specifically, the average threshold voltage, which is measured andprocessed from the test transfer transistors Tx1 t positioned at theleft tops in the respective unit pixel groups of the test pattern region17, may be compared with the threshold voltage value of the transfertransistors Tx1 positioned at the left tops in the respective unit pixelgroups of the active pixel region 15.

When the average threshold voltage is larger than the threshold voltagevalue which is preset, it may indicate that the actual thresholdvoltages of the transfer transistors Tx1 positioned at the left tops inthe respective unit pixel groups of the active pixel region 15 arelarger than the preset threshold voltage due to an error in process.Thus, during an actual operation, the offset may be compensated for bysupplying a bias larger than the reference bias to the transfertransistors Tx1 positioned at the left tops in the respective unit pixelgroups of the active pixel region 15. For example, referring to FIGS. 6and 7, the first correction bias Vp1 may be the first reference voltagefor turning on first transfer transistor Tx1+0.2V.

Furthermore, the average threshold voltage measured and processed usingthe test transfer transistors Tx2 t, which are positioned at the rightbottoms in the respective unit pixel groups of the test pattern region17, may be compared with a preset threshold voltage value of thetransfer transistors Tx2, which are positioned at the right bottoms inthe respective unit pixel groups of the active pixel region 15.

When the average threshold voltage is smaller than the preset thresholdvoltage value, it may indicate that the actual threshold voltages of thetransfer transistors Tx2, which are positioned at the right bottoms inthe respective unit pixel groups of the active pixel region 15, aresmaller than the preset threshold voltage due to an error in process.Thus, during an actual operation, the offset may be compensated for bysupplying a bias smaller than the reference bias to the transfertransistors Tx2, which are positioned at the right bottoms in therespective unit pixel groups of the active pixel region 15. For example,referring to FIGS. 6 and 7, the second correction bias Vp2 may be thesecond reference voltage for turning off second transfer transistorTx2−0.1V.

In accordance with the present embodiments, the image sensor may includethe active pixel region and the test pattern region which include pixelshaving substantially the same characteristics as pixels formed in theactive pixel region, and effectively correct an offset between pixels inthe active pixel region using electrical characteristics measured usingthe test pattern region.

Although various embodiments have been described for illustrativepurposes, it will be apparent to those skilled in the art that variouschanges and modifications may be made without departing from the spiritand scope of the invention as defined in the following claims.

What is claimed is:
 1. An image sensor comprising: a substrate includingan active pixel and a test pattern, wherein the test pattern is locatedadjacent to the active pixel, wherein the active pixel comprises a firstphotodiode, a floating diffusion, a first channel provided between thefirst photodiode and the floating diffusion, and a first transfer gateelectrode provided over the first channel, wherein the test patterncomprises a first test photodiode, a test floating diffusion, a secondchannel provided between the first test photodiode and the test floatingdiffusion, a first test transfer gate electrode provided over the secondchannel, and a first contact plug connected to the first testphotodiode, and wherein the first test photodiode, the test floatingdiffusion, the second channel, and the first test transfer gate havesubstantially the same alignment errors as the first photodiode, thefloating diffusion, the first channel, and the first transfer gateelectrode, respectively.
 2. The image sensor of claim 1, furthercomprising: a logic circuit adjacent to the active pixel; and acorrection circuit formed in the logic circuit, wherein the correctioncircuit is connected to the first transfer gate electrode.
 3. The imagesensor of claim 2, wherein the test pattern is formed between the logiccircuit and the active pixel.
 4. The image sensor of claim 2, whereinthe test pattern is connected to a side surface of the active pixel. 5.The image sensor of claim 2, wherein the first test photodiode, the testfloating diffusion, the second channel, and the first test transfer gateelectrode, in combination, form a test transfer transistor, wherein thecorrection circuit comprises a storage unit and a compensation unit,wherein the storage unit stores an electrical characteristic of the testtransfer transistor, a correction bias calculated based on theelectrical characteristic, or both; and wherein the compensation unit isconnected to the storage unit and provides the correction bias to thefirst transfer gate electrode.
 6. The image sensor of claim 5, whereinthe correction circuit further comprises: a calculation unit connectedto the storage unit and processing the electrical characteristic of thetest transfer transistor.
 7. The image sensor of claim 5, wherein thecorrection circuit further comprises: a comparison unit connectedbetween the storage unit and the compensation unit and calculates thecorrection bias by comparing the electrical characteristic with areference bias.
 8. The image sensor of claim 5, wherein the storage unitcomprises a nonvolatile memory.
 9. The image sensor of claim 1, whereinthe active pixel further comprises a second photodiode, a third channelprovided between the second photodiode and the floating diffusion, and asecond transfer gate electrode provided over the third channel, whereinthe floating diffusion is arranged between the first photodiode and thesecond photodiode, wherein the test pattern region further comprises asecond test photodiode, a fourth channel provided between the secondtest photodiode and the test floating diffusion, a second test transfergate electrode provided over the fourth channel, and a second contactplug connected to the second test photodiode, wherein the test floatingdiffusion is arranged between the first test photodiode and the secondtest photodiode, and wherein the second test photodiode, the fourthchannel, and the second test transfer gate electrode have substantiallythe same alignment errors as the second photodiode, the third channel,and the second transfer gate electrode, respectively.
 10. The imagesensor of claim 9, wherein the first channel has a first length, thesecond channel has a second length, the third channel has a thirdlength, and the fourth channel has a fourth length, wherein the secondlength of the second channel is substantially equal to the first lengthof the first channel, and wherein the fourth length of the fourthchannel is substantially equal to the third length of the third channel.11. The image sensor of claim 10, wherein the third length of the thirdchannel is smaller than the first length of the first channel.
 12. Theimage sensor of claim 9, further comprising: a third contact plugconnected to the test floating diffusion.
 13. The image sensor of claim9, wherein the first test photodiode, the second test photodiode, thetest floating diffusion, the second channel, the fourth channel, thefirst test transfer gate electrode, and the second test transfer gateelectrode comprise the same materials as the first photodiode, thesecond photodiode, the floating diffusion, the first channel, the thirdchannel, the first transfer gate electrode, and the second transfer gateelectrode, respectively.
 14. The image sensor of claim 1, furthercomprising: a color filter layer over the first photodiode; and amicrolens layer over the color filter layer.
 15. An offset correctionmethod of an image sensor having an active pixel region and a testpattern region, each of which includes a plurality of unit pixel groupsarranged in a two-dimensional manner, the unit pixel groups eachincluding a plurality of pixels sharing one floating diffusion, theoffset correction method comprising: measuring electricalcharacteristics of test transfer transistors in the test pattern region;processing electrical characteristics of test transfer transistorsformed at first positions in the unit pixel groups of the test patternregion; calculating correction values by comparing the processedelectrical characteristics to reference characteristics of transfertransistors formed at second positions in the unit pixel groups of theactive pixel region; and correcting offsets for the transfer transistorsformed at the second positions in the unit pixel groups of the activepixel region based on the calculated correction value.
 16. The offsetcorrection method of claim 15, wherein the first position and the secondposition in each of the unit pixel groups are the same positions. 17.The offset correction method of claim 15, wherein the active pixelregion comprises a first channel between a first photodiode and afloating diffusion and a first transfer gate electrode over the firstchannel, the test pattern region comprises a second channel between afirst test photodiode and a test floating diffusion, a first testtransfer gate electrode over the second channel, and a first contactplug connected to the first test photodiode, and the first testphotodiode, the test floating diffusion, the second channel and thefirst test transfer gate electrode have substantially the same alignmenterrors as the first photodiode, the floating diffusion, the firstchannel and the first transfer gate electrodes.